1. Field of the Invention
The present invention relates to a grease composition for use in a pivot assembly bearing, more particularly, relates to a grease composition for a pivot assembly bearing which serves in providing a low torque at the beginning of the driving and is excellent in outgas properties and scattering properties, and further, relates to a grease composition for a pivot assembly bearing not including a lead compound, and a bearing for pivot assembly sealed therewith.
2. Description of Background Art
A grease composition has been known which comprises a base oil and a thickener including a diurea compound, and which contains a zinc compound in an amount of 0.1 to 10% by mass based on the total amount of the grease, wherein a diurea compound having an aromatic ring-containing hydrocarbon group having 7 to 12 carbon atoms and a hydrocarbon group having 8 to 20 carbon atoms is used, which is included in an amount of 10 to 30% by mass based on the total amount of the grease such that a value of (number of moles of the aromatic ring-containing hydrocarbon group/(number of moles of the aromatic ring-containing hydrocarbon group+number of moles of the hydrocarbon group)) becomes 0.50 to 1.0 (JP-A No. 2004-323586).
However, it is disclosed that this grease composition is suited for rolling bearings to be incorporated in electrical components of automobiles, engine components used under high-temperature, high-speed and high-load conditions such as alternators and middle pulleys, and electromagnetic clutches for car air-conditioners, or components brought into contact with water such as water pumps, and that hydrocarbon-based oil, mineral oil, ester-based oil, ether-based oil or the like may be used as a base oil.
Furthermore, in grease compositions for sealing the bearings used in computers, apparatuses for manufacturing semiconductors and the like, a low-noise and low-torque grease as typified by greases including a lithium soap as a thickener and an ester oil as a base oil has been used in case where importance is placed on low noise properties and low vibration properties. However, this lithium soap-ester oil-based grease is apt to be scattered, therefore, there is a highly possibility for staining of recording media when it is used without modification (Japanese Patent No. 3,324,628).
Moreover, known greases for use in pivot assembly bearings include a grease developed by the present applicant comprising a mixed base oil including a mineral oil and poly α-olefin, and a diurea compound having an alicyclic hydrocarbon group having 6 to 12 carbon atoms and an aliphatic hydrocarbon group having 8 to 22 carbon atoms as a thickener.
However, this grease composition is disadvantageous in terms of torque properties in a low temperature range and is difficult to suit for environment because of addition of an organic lead compound.